At elevated feed water pressures, polymeric filtration membranes can be damaged internally by what can be referred to as “internal fouling.” Elevated pressures can become necessary for reverse osmosis (RO) membrane processes to overcome the effects of internal fouling in order to maintain water flux flow rates. Further increases of applied pressure can be required with time due to fouling for example by contaminants which occur on the membrane surface and may be considered “external” or surface fouling. Surface fouling, and the resulting higher operating pressures required to maintain a desired flux may lead to additional internal fouling, of the membrane. Such increased internal fouling, in turn, can require even higher operating pressures to achieve a desired flux. Although surface deposits that lead to elevated operating pressure can be removed by physical and chemical cleaning methods, internal fouling due to high pressure physical compaction cannot be reversed. Such irreversible fouling of RO membranes can also lead to higher long-term operating cost and energy consumption.
For example, conventional RO membranes typically exhibit a first or initial permeate flow rate at a first applied pressure which decreases with time as internal fouling occurs, typically in a matter of days or hours, to a lower permeate flow rate at the same applied pressure until a generally steady state permeate flow is achieved. After the steady state permeate flow rate is achieved, the flux flow rate may further decline as a result of external fouling. Most or all of the reduced flux flow may be resulting from external fouling may be recovered by various cleaning processes which typically remove the surface contaminants. As a result, whether applied pressure is increased to maintain a desired permeate flow in light of internal and external fouling, or the applied pressure is maintained constant and the permeate flow rate allowed to decrease from an initial state to a steady state due to irreversible internal fouling and further due to reversible, or at least partially reversible external fouling, fouling is a major operating problem for conventional membranes such as RO membranes.
Therefore, there remains a need for methods and compositions that overcome these deficiencies and that effectively provide for membranes having improved membrane resistance to internal and/or external fouling and improved or reduced water flux decline over time from initial to steady state conditions, while maintaining satisfactory water permeability and solute rejection.